1. Field of the Invention
The present invention relates to a vehicle front and rear wheels drive system and a clutch changeover method.
2. Description of the Related Art
In conventional four-wheel drive vehicles (hereinafter, referred to as “4WD vehicles”), when turning a corner having a small turning radius at low to intermediate speeds in four-wheel drive running conditions, there occurs a tight corner braking phenomenon due to the generation of a turning speed difference between front and rear wheels attributed to a difference in turning radius. Generally known as a technology for solving the problem of the tight corner braking phenomenon is a front and rear wheels drive system such as disclosed in JP-B-7-64219. The front and rear wheels drive system so disclosed is such that the average wheel speed of the secondary drive wheels is adjusted relative to the average wheel speed of the primary drive wheels by providing a change-speed apparatus between the primary drive wheels and the secondary drive wheels. Specifically speaking, this change-speed apparatus is designed to change over a direct coupled state in which the average wheel speed of the primary drive wheels and the average wheel speed of the secondary drive wheels are substantially equal and a speed increased state in which the average wheel speed of the secondary drive wheels becomes faster than the average wheel speed of the primary drive wheels by switching ON and OFF a direct coupling clutch and a change-speed clutch. Thus, according to the front and rear wheels drive system so constructed, when turning a tight corner in the four-wheel drive running conditions, the occurrence of the tight corner braking phenomenon is prevented by putting the secondary-drive wheels in the speed increased state using the change-speed apparatus.
In the change-speed apparatus of the front and rear wheels drive system, since at least two hydraulic or electromagnetic actuators are needed as a power source for operating the direct coupling clutch and the change-speed clutch, there is caused a defect that the size and weight of the apparatus are increased.